Simply not a factor in calculating the effect of a collision. Velocity is velocity. If two cars collide head-on at 60mph it doesn't matter whether they had snow tyres or radials. Consider the biomechanical forces involved — velocity is increased (players travel faster on skates than on foot) so deceleration would be increased — increasing G-force of impact. It's a bit like the argument between proponents of heavy baseball bats vs lighter back in the day. The heavy bat argument simply could not understand that it was bat speed at contact that made the ball travel further.